Enhanced interface thermoelectric coolers with all-metals tips

ABSTRACT

A thermoelectric device with improved efficiency is provided. In one embodiment, the thermoelectric device includes a first thermoelement and a second thermoelement electrically coupled to the first thermoelement. An array of first tips are in close physical proximity to, but not necessarily in physical contact with, the first thermoelement at a first set of discrete points. An array of second tips are in close physical proximity to, but not necessarily in physical contact with, the second thermoelement at a second set of discrete points. The first and second conical are constructed entirely from metal, thus reducing parasitic resistances.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/731,616,filed Dec. 7, 2000 now U.S. Pat. No. 6,403,876, status allowed.

The present application is related to co-pending U.S. patent applicationSer. No. 09/731,997 (IBM Docket No. AUS9-2000-0415-US1) entitled“THERMOELECTRIC COOLERS WITH ENHANCED STRUCTURED INTERFACES” filed onDec. 7, 2000, to co-pending U.S. patent application Ser. No. 09/731,999(IBM Docket No. AUS9-2000-0564-US1) entitled “COLD POINT DESIGN FOREFFICIENT THERMOELECTRIC COOLERS” filed on Dec. 7, 2000, and toco-pending U.S. patent application Ser. No. 09/731,614 (IBM Docket No.AUS9-2000-0556-US1) entitled “ENHANCED INTERFACE THERMOELECTRIC COOLERSUSING ETCHED THERMOELECTRIC MATERIAL TIPS” filed on Dec. 7, 2000. Thecontent of the above mentioned commonly assigned, co-pending U.S. Patentapplications are hereby incorporated herein by reference for allpurposes.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to devices for cooling substances such as,for example, integrated circuit chips, and more particularly, thepresent invention relates to thermoelectric coolers.

2. Description of Related Art

As the speed of computers continues to increase, the amount of heatgenerated by the circuits within the computers continues to increase.For many circuits and applications, increased heat degrades theperformance of the computer. These circuits need to be cooled in orderto perform most efficiently. In many low end computers, such as personalcomputers, the computer may be cooled merely by using a fan and fins forconvective cooling. However, for larger computers, such as main frames,that perform at faster speeds and generate much more heat, thesesolutions are not viable.

Currently, many main frames utilize vapor compression coolers to coolthe computer. These vapor compression coolers perform essentially thesame as the central air conditioning units used in many homes. However,vapor compression coolers are quite mechanically complicated requiringinsulation and hoses that must run to various parts of the main frame inorder to cool the particular areas that are most susceptible todecreased performance due to overheating.

A much simpler and cheaper type of cooler are thermoelectric coolers.Thermoelectric coolers utilize a physical principle known as the PeltierEffect, by which DC current from a power source is applied across twodissimilar materials causing heat to be absorbed at the junction of thetwo dissimilar materials. Thus, the heat is removed from a hot substanceand may be transported to a heat sink to be dissipated, thereby coolingthe hot substance. Thermoelectric coolers may be fabricated within anintegrated circuit chip and may cool specific hot spots directly withoutthe need for complicated mechanical systems as is required by vaporcompression coolers.

However, current thermoelectric coolers are not as efficient as vaporcompression coolers requiring more power to be expended to achieve thesame amount of cooling. Furthermore, current thermoelectric coolers arenot capable of cooling substances as greatly as vapor compressioncoolers. Therefore, a thermoelectric cooler with improved efficiency andcooling capacity would be desirable so that complicated vaporcompression coolers could be eliminated from small refrigerationapplications, such as, for example, main frame computers, thermalmanagement of hot chips, RF communication circuits, magnetic read/writeheads, optical and laser devices, and automobile refrigeration systems.

SUMMARY OF THE INVENTION

The present invention provides a thermoelectric device with improvedefficiency. In one embodiment, the thermoelectric device includes afirst thermoelement and a second thermoelement electrically coupled tothe first thermoelement. An array of first tips are in close physicalproximity to, but not necessarily in physical contact with, the firstthermoelement at a first set of discrete points. An array of second tipsare in close physical proximity to, but not necessarily in physicalcontact with, the second thermoelement at a second set of discretepoints. The first and second conical are constructed entirely frommetal, thus reducing parasitic resistances.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a high-level block diagram of a Thermoelectric Cooling(TEC) device in accordance with the prior art;

FIG. 2 depicts a cross sectional view of a thermoelectric cooler withenhanced structured interfaces in accordance with the present invention;

FIG. 3 depicts a planer view of thermoelectric cooler 200 in FIG. 2 inaccordance with the present invention;

FIGS. 4A and 4B depicts cross sectional views of tips that may beimplemented as one of tips 250 in FIG. 2 in accordance with the presentinvention;

FIG. 5 depicts a cross sectional view illustrating the temperature fieldof a tip near to a superlattice in accordance with the presentinvention;

FIG. 6 depicts a cross sectional view of a thermoelectric cooler withenhanced structured interfaces with all metal tips in accordance withthe present invention;

FIG. 7 depicts a cross-sectional view of a sacrificial silicon templatefor forming all metal tips in accordance with the present invention;

FIG. 8 depicts a flowchart illustrating an exemplary method of producingall metal cones using a silicon sacrificial template in accordance withthe present invention;

FIG. 9 depicts a cross sectional view of all metal cones formed usingpatterned photoresist in accordance with the present invention;

FIG. 10 depicts a flowchart illustrating an exemplary method of formingall metal cones using photoresist in accordance with the presentinvention;

FIG. 11 depicts a cross-sectional view of a thermoelectric cooler withenhanced structural interfaces in which the thermoelectric materialrather than the metal conducting layer is formed into tips at theinterface in accordance with the present invention;

FIG. 12 depicts a flowchart illustrating an exemplary method offabricating a thermoelectric cooler in accordance with the presentinvention;

FIG. 13 depicts a cross-sectional diagram illustrating the positioningof photoresist necessary to produce tips in a thermoelectric material;

FIG. 14 depicts a diagram showing a cold point tip above a surface foruse in a thermoelectric cooler illustrating the positioning of the tiprelative to the surface in accordance with the present invention; and

FIG. 15 depicts a schematic diagram of a thermoelectric power generator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the figures and, in particular, with reference toFIG. 1, a high-level block diagram of a Thermoelectric Cooling (TEC)device is depicted in accordance with the prior art. Thermoelectriccooling, a well known principle, is based on the Peltier Effect, bywhich DC current from power source 102 is applied across two dissimilarmaterials causing heat to be absorbed at the junction of the twodissimilar materials. A typical thermoelectric cooling device utilizesp-type semiconductor 104 and n-type semiconductor 106 sandwiched betweenpoor electrical conductors 108 that have good heat conductingproperties. N-type semiconductor 106 has an excess of electrons, whilep-type semiconductor 104 has a deficit of electrons.

As electrons move from electrical conductor 110 to n-type semiconductor106, the energy state of the electrons is raised due to heat energyabsorbed from heat source 112. This process has the effect oftransferring heat energy from heat source 112 via electron flow throughn-type semiconductor 106 and electrical conductor 114 to heat sink 116.The electrons drop to a lower energy state and release the heat energyin electrical conductor 114.

The coefficient of performance, η, of a cooling refrigerator, such asthermoelectric cooler 100, is the ratio of the cooling capacity of therefrigerator divided by the total power consumption of the refrigerator.Thus the coefficient of performance is given by the equation:$\eta = \frac{{\alpha \quad {IT}_{c}} - {\frac{1}{2}I^{2}R} - {K\quad \Delta \quad T}}{{I^{2}R} + {\alpha \quad I\quad \Delta \quad T}}$

where the term αIT_(c) is due to the thermoelectric cooling, the term½I²R is due to Joule heating backflow, the term KΔT is due to thermalconduction, the term I²R is due to Joule loss, the term αIΔT is due towork done against the Peltier voltage, α is the Seebeck coefficient forthe material, T_(c) is the temperature of the heat source, and ΔT is thedifference in the temperature of the heat source form the temperature ofthe heat sink.

The maximum coefficient of performance is derived by optimizing thecurrent, I, and is given by the following relation:$\eta_{\max} = {{{\left( \frac{T_{c}}{\Delta \quad T} \right)\left\lbrack \frac{\gamma - \frac{T_{h}}{T_{c}}}{\gamma + 1} \right\rbrack}\quad {where}\quad \gamma} = {{\sqrt{1 + {\frac{\alpha^{2}\sigma}{\lambda}\left( \frac{T_{h} + T_{c}}{2} \right)}}\quad {and}\quad ɛ} = \frac{\gamma - \frac{T_{h}}{T_{c}}}{\gamma + 1}}}$

where ε is the efficiency factor of the refrigerator. The figure ofmerit, ZT, is given by the equation:${ZT} = \frac{\alpha^{2}\sigma \quad T}{\lambda}$

where λ is composed of two components: λ_(e), the component due toelectrons, and λ_(L), the component due to the lattice. Therefore, themaximum efficiency, ε, is achieved as the figure of merit, ZT,approaches infinity. The efficiency of vapor compressor refrigerators isapproximately 0.3. The efficiency of conventional thermoelectriccoolers, such as thermoelectric cooler 100 in FIG. 1, is typically lessthan 0.1. Therefore, to increase the efficiency of thermoelectriccoolers to such a range as to compete with vapor compressionrefrigerators, the figure of merit, ZT, must be increased to greaterthan 2. If a value for the figure of merit, ZT, of greater than 2 can beachieved, then the thermoelectric coolers may be staged to achieve thesame efficiency and cooling capacity as vapor compression refrigerators.

With reference to FIG. 2, a cross sectional view of a thermoelectriccooler with enhanced structured interfaces is depicted in accordancewith the present invention. Thermoelectric cooler 200 includes a heatsource 226 from which, with current I flowing as indicated, heat isextracted and delivered to heat sink 202. Heat source 226 may bethermally coupled to a substance that is desired to be cooled. Heat sink202 may be thermally coupled to devices such as, for example, a heatpipe, fins, and/or a condensation unit to dissipate the heat removedfrom heat source 226 and/or further cool heat source 226.

Heat source 226 is comprised of p− type doped silicon. Heat source 226is thermally coupled to n+ type doped silicon regions 224 and 222 oftips 250. N+ type regions 224 and 222 are electrical conducting as wellas being good thermal conductors. Each of N+ type regions 224 and 222forms a reverse diode with heat source 226 such that no current flowsbetween heat source 226 and n+regions 224 and 222, thus providing theelectrical isolation of heat source 226 from electrical conductors 218and 220.

Heat sink 202 is comprised of p− type doped silicon. Heat sink 202 isthermally coupled to n+ type doped silicon regions 204 and 206. N+ typeregions 204 and 206 are electrically conducting and good thermalconductors. Each of N+ type regions 204 and 206 and heat sink 202 formsa reverse diode so that no current flows between the N+ type regions 204and 206 and heat sink 202, thus providing the electrical isolation ofheat sink 202 from electrical conductor 208. More information aboutelectrical isolation of thermoelectric coolers may be found in commonlyU.S. patent application Ser. No. 09/458,270 entitled {ElectricallyIsolated Ultra-Thin Substrates for Thermoelectric Coolers” (IBM DocketNo. AUS9-99-0413-US1) assigned to the International Business MachinesCorporation of Armonk, N.Y. and filed on Dec. 9, 1999, the contents ofwhich are hereby incorporated herein for all purposes.

The need for forming reverse diodes with n+ and p− regions toelectrically isolate conductor 208 from heat sink 202 and conductors 218and 220 from heat source 226 is not needed if the heat sink 202 and heatsource 226 are constructed entirely from undoped non-electricallyconducting silicon. However, it is very difficult to ensure that thesilicon is entirely undoped. Therefore, the presence of the reversediodes provided by the n+ and p− regions ensures that heat sink 202 andheat source 226 are electrically isolated from conductors 208, 218, and220. Also, it should be noted that the same electrical isolation usingreverse diodes may be created other ways, for example, by using p+typedoped silicon and n− type doped silicon rather than the p− and n+ typesdepicted. The terms n+ and p+, as used herein, refer to highly n dopedand highly p doped semiconducting material respectively. The terms n−and p−, as used herein, mean lightly n doped and lightly p dopedsemiconducting material respectively.

Thermoelectric cooler 200 is similar in construction to thermoelectriccooler 100 in FIG. 1. However, N-type 106 and P-type 104 semiconductorstructural interfaces have been replaced with superlattice thermoelementstructures 210 and 212 that are electrically coupled by electricalconductor 208. Electrical conductor 208 may be formed from platinum (Pt)or, alternatively, from other conducting materials, such as, forexample, tungsten (W), nickel (Ni), or titanium copper nickel (Ti/Cu/Ni)metal films.

A superlattice is a structure consisting of alternating layers of twodifferent semiconductor materials, each several nanometers thick.Thermoelement 210 is constructed from alternating layers of N-typesemiconducting materials and the superlattice of thermoelement 212 isconstructed from alternating layers of P-type semiconducting materials.Each of the layers of alternating materials in each of thermoelements210 and 212 is 10 nanometers (nm) thick. A superlattice of twosemiconducting materials has lower thermal conductivity, λ, and the sameelectrical conductivity, σ, as an alloy comprising the same twosemiconducting materials.

In one embodiment, superlattice thermoelement 212 comprises alternatinglayers of p-type bismuth chalcogenide materials such as, for example,alternating layers of Bi₂Te₃/Sb₂Te₃ with layers of Bi_(0.5)Sb_(1.5)Te₃,and the superlattice of thermoelement 210 comprises alternating layersof n-type bismuth chalcogenide materials, such as, for example,alternating layers of Bi₂Te₃ with layers of Bi₂Se₃. Other types ofsemiconducting materials may be used for superlattices forthermoelements 210 and 212 as well. For example, rather than bismuthchalcogenide materials, the superlattices of thermoelements 210 and 212may be constructed from cobalt antimony skutteridite materials.

Thermoelectric cooler 200 also includes tips 250 through whichelectrical current I passes into thermoelement 212 and then fromthermoelement 210 into conductor 218. Tips 250 includes n+ typesemiconductor 222 and 224 formed into pointed conical structures with athin overcoat layer 218 and 220 of conducting material, such as, forexample, platinum (Pt). Other conducting materials that may be used inplace of platinum include, for example, tungsten (W), nickel (Ni), andtitanium copper nickel (Ti/Cu/Ni) metal films. The areas between andaround the tips 250 and thermoelectric materials 210 and 212 should beevacuated or hermetically sealed with a gas such as, for example, drynitrogen.

On the ends of tips 250 covering the conducting layers 218 and 220 is athin layer of semiconducting material 214 and 216. Layer 214 is formedfrom a P-type material having the same Seebeck coefficient, α, as thenearest layer of the superlattice of thermoelement 212 to tips 250.Layer 216 is formed from an N-type material having the same Seebeckcoefficient, α, as the nearest layer of thermoelement 210 to tips 250.The P-type thermoelectric overcoat layer 214 is necessary forthermoelectric cooler 200 to function since cooling occurs in the regionnear the metal where the electrons and holes are generated. The n-typethermoelectric overcoat layer 216 is beneficial, because maximum coolingoccurs where the gradient (change) of the Seebeck coefficient ismaximum. The thermoelectric overcoat 214 for the P-type region isapproximately 60 nm thick. A specific thickness of the n-typethermoelectric overcoat 216 has yet to be fully refined, but it isanticipated that it should be in a similar thickness range to thethickness of the thermoelectric overcoat 214.

By making the electrical conductors, such as, conductors 110 in FIG. 1,into pointed tips 250 rather than a planer interface, an increase incooling efficiency is achieved. Lattice thermal conductivity, λ, at thepoint of tips 250 is very small because of lattice mismatch. Forexample, the thermal conductivity, λ, of bismuth chalcogenides isnormally approximately 1 Watt/meter*Kelvin. However, in pointed tipstructures, such as tips 250, the thermal conductivity is reduced, dueto lattice mismatch at the point, to approximately 0.2Watts/meter*Kelvin. However, the electrical conductivity of thethermoelectric materials remains relatively unchanged. Therefore, thefigure of merit, ZT, may increased to greater than 2.5 for this kind ofmaterial. Another type of material that is possible for thesuperlattices of thermoelements 210 and 212 is cobalt antimonyskutteridites. These type of materials typically have a very highthermal conductivity, λ, making them normally undesirable. However, byusing the pointed tips 250, the thermal conductivity can be reduced to aminimum and produce a figure of merit, ZT, for these materials ofgreater than 4, thus making these materials very attractive for use inthermoelements 210 and 212. Therefore, the use of pointed tips 250further increases the efficiency of the thermoelectric cooler 200 suchthat it is comparable to vapor compression refrigerators.

Another advantage of the cold point structure is that the electrons areconfined to dimensions smaller than the wavelength (corresponding totheir kinetic energy). This type of confinement increases the localdensity of states available for transport and effectively increases theSeebeck coefficient. Thus, by increasing α and decreasing λ, the figureof merit ZT is increased.

Normal cooling capacity of conventional thermoelectric coolers, such asillustrated in FIG. 1, are capable of producing a temperaturedifferential, ΔT, between the heat source and the heat sink of around 60Kelvin. However, thermoelectric cooler 200 is capable of producing atemperature differential on the order of 150 Kelvin. Thus, with twothermoelectric coolers coupled to each other, cooling to temperatures inthe range of liquid Nitrogen (less than 100 Kelvin) is possible.However, different materials may need to be used for thermoelements 210and 212. For example, bismuth telluride has a very low α at lowtemperature (i.e. less than −100 degrees Celsius). However, bismuthantimony alloys perform well at low temperature.

Another advantage of the cobalt antimony skutteridite materials over thebismuth chalcogenide materials, not related to temperature, is the factthe cobalt antimony skutteridite materials are structurally more stablewhereas the bismuth chalcogenide materials are structurally weak.

Those of ordinary skill in the art will appreciate that the constructionof the thermoelectric cooler in FIG. 2 may vary depending on theimplementation. For example, more or fewer rows of tips 250 may beincluded than depicted in FIG. 1. The depicted example is not meant toimply architectural limitations with respect to the present invention.

With reference now to FIG. 3, a planer view of thermoelectric cooler 200in FIG. 2 is depicted in accordance with the present invention.Thermoelectric cooler 300 includes an n-type thermoelectric materialsection 302 and a p-type thermoelectric material section 304. Bothn-type section 302 and p-type section 304 include a thin layer ofconductive material 306 that covers a silicon body.

Section 302 includes an array of conical tips 310 each covered with athin layer of n-type material 308 of the same type as the nearest layerof the superlattice for thermoelement 210. Section 304 includes an arrayof conical tips 312 each covered with a thin layer of p-type material314 of the same type as the nearest layer of the superlattice forthermoelement 212.

With reference now to FIGS. 4A and 4B, a cross sectional views of tipsthat may be implemented as one of tips 250 in FIG. 2 is depicted inaccordance with the present invention. Tip 400 includes a silicon conethat has been formed with a cone angle of approximately 35 degrees. Athin layer 404 of conducting material, such as platinum (Pt), overcoatsthe silicon 402. A thin layer of thermoelectric material 406 covers thevery end of the tip 400. The cone angle after all layers have beendeposited is approximately 45 degrees. The effective tip radius of tip400 is approximately 50 nanometers.

Tip 408 is an alternative embodiment of a tip, such as one of tips 250.Tip 408 includes a silicon cone 414 with a conductive layer 412 andthermoelectric material layer 410 over the point. However, tip 408 has amuch sharper cone angle than tip 400. The effective tip radius of tip408 is approximately 10 nanometers. It is not known at this time whethera broader or narrower cone angle for the tip is preferable. In thepresent embodiment, conical angles of 45 degrees for the tip, asdepicted in FIG. 4A, have been chosen, since such angle is in the middleof possible ranges of cone angle and because such formation is easilyformed with silicon with a platinum overcoat. This is because a KOH etchalong the 100 plane of silicon naturally forms a cone angle of 54degrees. Thus, after the conductive and thermoelectric overcoats havebeen added, the cone angle is approximately 45 degrees.

With reference now to FIG. 5, a cross sectional view illustrating thetemperature field of a tip near to a superlattice is depicted inaccordance with the present invention. Tip 504 may be implemented as oneof tips 250 in FIG. 2. Tip 504 has a effective tip radius, a, of 30-50nanometers. Thus, the temperature field is localized to a very smalldistance, r, approximately equal to 2a or around 60-100 nanometers.Therefore, a superlattice 502 need to be only a few layers thick with athickness, d, of around 100 nanometers. Therefore, using pointed tips, athermoelectric cooler with only 5-10 layers for the superlattice issufficient.

Thus, fabricating a thermoelectric cooler, such as, for example,thermoelectric cooler 200, is not extremely time consuming, since only afew layers of the superlattice must be formed rather than numerouslayers which can be very time consuming. Thus, thermoelectric cooler 200can be fabricated very thin (on the order of 100 nanometers thick) ascontrasted to prior art thermoelectric coolers which were on the orderof 3 millimeters or greater in thickness.

Other advantages of a thermoelectric cooler with pointed tip interfacesin accordance with the present invention include minimization of thethermal conductivity of the thermoelements, such as thermoelements 210and 212 in FIG. 2, at the tip interfaces. Also, thetemperature/potential drops are localized to an area near the tips,effectively achieving scaling to sub-100-nanometer lengths. Furthermore,using pointed tips minimizes the number layers for superlattice growthby effectively reducing the thermoelement lengths. The present inventionalso permits electrodeposition of thin film structures and avoidsflip-chip bonds. The smaller dimensions allow for monolithic integrationof n-type and p-type thermoelements.

The thermoelectric cooler of the present invention may be utilized tocool items, such as, for example, specific spots within a main framecomputer, lasers, optic electronics, photodetectors, and PCR ingenetics.

With reference now to FIG. 6, a cross sectional view of a thermoelectriccooler with enhanced structured interfaces with all metal tips isdepicted in accordance with the present invention. Although the presentinvention has been described above as having tips 250 constructed fromsilicon cones constructed from the n+ semiconducting regions 224 and222, tips 250 in FIG. 2 may be replaced by tips 650 as depicted in FIG.6. Tips 650 have all metal cones 618 and 620. In the depictedembodiment, cones 618 and 620 are constructed from copper and have anickel overcoat layer 660 and 662. Thermoelectric cooler 600 isidentical to thermoelectric cooler 200 in all other respects, includinghaving a thermoelectric overcoat 216 and 214 over the tips 650.Thermoelectric cooler 600 also provides the same benefits asthermoelectric cooler 200. However, by using all metal cones rather thansilicon cones covered with conducting material, the parasiticresistances within the cones become very low, thus further increasingthe efficiency of thermoelectric cooler 600 over the already increasedefficiency of thermoelectric cooler 200. The areas surrounding tips 650and between tips 650 and thermoelectric materials 210 and 212 should bevacuum or hermetically sealed with a gas, such as, for example, drynitrogen.

Also, as in FIG. 2, heat source 226 is comprised of p− type dopedsilicon. In contrast to FIG. 2, however, heat source 226 is thermallycoupled to n+ type doped silicon regions 624 and 622 that do not formpart of the tipped structure 650 rather than to regions that do formpart of the tipped structure as do regions 224 and 222 do in FIG. 2. N+type doped silicon regions 624 and 622 do still perform the electricalisolation function performed by regions 224 and 222 in FIG. 2.

Several methods may be utilized to form the all metal cones as depictedin FIG. 6. For example, with reference now to FIG. 7, a cross-sectionalview of a sacrificial silicon template that may be used for forming allmetal tips is depicted in accordance with the present invention. Afterthe sacrificial silicon template 702 has been constructed having conicalpits, a layer of metal may be deposited over the template 702 to produceall metal cones 704. All metal cones 704 may then be used inthermoelectric cooler 600.

With reference now to FIG. 8, a flowchart illustrating an exemplarymethod of producing all metal cones using a silicon sacrificial templateis depicted in accordance with the present invention. To begin, conicalpits are fabricated by anisotropic etching of silicon to create a mold(step 802). This may be done by a combination of KOH etching, oxidation,and/or focused ion-beam etching. Such techniques of fabricating asilicon cone are well known in the art.

The silicon sacrificial template is then coated with a thin sputteredlayer of seed metal, such as, for example, titanium or platinum (step804). Titanium is preferable since platinum forms slightly more roundedtips than titanium, which is very conforming to the conical pits. Next,copper is electrochemically deposited to fill the valleys (conical pits)in the sacrificial silicon template. (step 806). The top surface of thecopper is then planarized (step 808). Methods of planarizing a layer ofmetal are well known in the art. The silicon oxide (SiO₂) substrate isthen removed by selective etching methods well known in the art (step810). The all metal cones produced in this manner may then be coveredwith a coat of another metal, such as, for example, nickel or titaniumand then with an ultra-thin layer of thermoelectric material. The nickelor titanium overcoat aids in electrodeposition of the thermoelectricmaterial overcoat.

One advantage to this method of producing all metal cones is that themold that is produced is reusable. The mold may be reused up to around10 times before the mold degrades and becomes unusable. Forming atemplate in this manner is very well controlled and produces veryuniform all metal conical tips since silicon etching is very predictableand can calculate slopes of the pits and sharpness of the cones producedto a very few nanometers.

Other methods of forming all metal cones may be used as well. Forexample, with reference now to FIG. 9, a cross sectional view of allmetal cones 902 formed using patterned photoresist is depicted inaccordance with the present invention. In this method, a layer of metalis formed over the bottom portions of a partially fabricatedthermoelectric cooler. A patterned photoresist 904-908 is then used tofashion all metal cones 902 with a direct electrochemical etchingmethod.

With reference now to FIG. 10, a flowchart illustrating an exemplarymethod of forming all metal cones using photoresist is depicted inaccordance with the present invention. To begin, small sections ofphotoresist are patterned over a metal layer, such as copper, of apartially fabricated thermoelectric cooler, such as thermoelectriccooler 600, in FIG. 6 (step 1002). The photoresist may be patterned inan array of sections having photoresist wherein each area of photoresistwithin the array corresponds to areas in which tips to the all metalcones are desired to be formed. The metal is then directly etchedelectrochemically (step 1004) to produce cones 902 as depicted in FIG.9. The photoresist is then removed and the tips of the all metal conesmay then be coated with another metal, such as, for example, nickel(step 1006). The second metal coating over the all metal cones may thenbe coated with an ultra-thin layer of thermoelectric material (step1008). Thus, all metal cones with a thermoelectric layer on the tips maybe formed which may used in a thermoelectric device, such as, forexample, thermoelectric cooler 600. The all metal conical pointsproduced in this manner are not as uniform as those produced using themethod illustrated in FIG. 8. However, this method currently is cheaperand therefore, if cost is an important factor, may be a more desirablemethod.

The depicted methods of fabricating all metal cones are merely examples.Other methods may be used as well to fabricate all metal cones for usewith thermoelectric coolers. Furthermore, other types of metals may beused for the all metal cone other than copper.

With reference now to FIG. 11, a cross-sectional view of athermoelectric cooler with enhanced structural interfaces in which thethermoelectric material rather than the metal conducting layer is formedinto tips at the interface is depicted in accordance with the presentinvention. Thermoelectric cooler 1100 includes a cold plate 1116 and ahot plate 1102, wherein the cold plate is in thermal contact with thesubstance that is to be cooled. Thermal conductors 1114 and 1118 providea thermal couple between electrical conducting plates 1112 and 1120respectively. Thermal conductors 1114 and 1118 are constructed ofheavily n doped (n+) semiconducting material that provides electricalisolation between cold plate 1116 and conductors 1112 and 1120 byforming reverse biased diodes with the p− material of the cold plate1116. Thus, heat is transferred from the cold plate 1116 throughconductors 1112 and 1120 and eventually to hot plate 1102 from which itcan be dissipated without allowing an electrical coupling between thethermoelectric cooler 1100 and the substance that is to be cooled.Similarly, thermal conductor 1104 provides a thermal connection betweenelectrical conducting plate 1108 and hot plate 1102, while maintainingelectrical isolation between the hot plate and electrical conductingplate 1108 by forming a reverse biased diode with the hot plate 1102 p−doped semiconducting material as discussed above. Thermal conductors1104 and 1106 are also an n+ type doped semiconducting material.Electrical conducting plates 1108, 1112, and 1120 are constructed fromplatinum (Pt) in this embodiment. However, other materials that are bothelectrically conducting and thermally conducting may be utilized aswell. Also, it should be mentioned that the areas surrounding tips1130-1140 and between tips 1130-1140 and thermoelectric materials 1122and 1110 should be evacuated to produce a vacuum or should behermetically sealed with a gas, such as, for example, dry nitrogen.

In this embodiment, rather than providing contact between thethermoelements and the heat source (cold end) metal electrode(conductor) through an array of points in the metal electrode as inFIGS. 2 and 6, the array of points of contact between the thermoelementand the metal electrode is provided by an array of points 1130-1140 inthe thermoelements 1124 and 1126. In the embodiments described abovewith reference to FIGS. 2 and 6, the metal electrode at the cold end wasformed over silicon tips or alternatively metal patterns were directlyetched to form all-metal tips. However, these methods requiredthermoelectric materials to be deposited over the cold and the hotelectrodes by electrochemical methods. The electrodeposited materialstend to be polycrystalline and do not have ultra-planar surfaces. Also,the surface thermoelectric properties may or may not be superior tosingle crystalline thermoelectric materials. Annealing improves thethermoelectric properties of the polycrystalline materials, but surfacesmoothness below 100 nm roughness levels remains a problem. The tips1130-1140 of the present embodiment may be formed from single crystal orpolycrystal thermoelectric materials by electrochemical etching.

In one embodiment, thermoelement 1124 is comprised of a super lattice ofsingle crystalline Bi₂Te₃/Sb₂Te₃ and Bi_(0.5)Sb_(1.5)Te₃ andthermoelement 1126 is formed of a super lattice of single crystallineBi₂Te₃/Bi₂Se₃ and Bi₂Te_(2.0)Se_(0.1). Electrically conducting plate1120 is coated with a thin layer 1122 of the same thermoelectricmaterial as is the material of the tips 1130-1134 that are nearest thinlayer 1120. Electrically conducting plate 1112 is coated with a thinlayer 1110 of the same thermoelectric material as is the material of thetips 1136-1140 that are nearest thin layer 1112.

With reference now to FIG. 12, a flowchart illustrating an exemplarymethod of fabricating a thermoelectric cooler, such as, for example,thermoelectric cooler 1100 in FIG. 11, is depicted in accordance withthe present invention. Optimized single crystal material are firstbonded to metal electrodes by conventional means or metal electrodes aredeposited onto single crystal materials to form the electrode connectionpattern (step 1202). The other side of the thermoelectric material 1314is then patterned (step 1204) by photoresist 1302-1306 as depicted inFIG. 13 and metal electrodes are used in an electrochemical bath as ananode to electrochemically etch the surface (step 1206). The tips1308-1312 as depicted in FIG. 13 are formed by controlling and stoppingthe etching process at appropriate times.

A second single crystal substrate is thinned by chemical-mechanicalpolishing and then electrochemically etching the entire substrate tonanometer films (step 1210). The second substrate with the ultra-thinsubstrate forms the cold end and the two substrates (the one with theultra-thin thermoelectric material and the other with the thermoelectrictips) are clamped together with pressure (step 1212). This structureretains high crystallinity in all regions other than the interface atthe tips. Also, the same method can be used to fabricate polycrystallinestructures rather than single crystalline structures.

With reference now to FIG. 14, a diagram showing a cold point tip abovea surface for use in a thermoelectric cooler illustrating thepositioning of the tip relative to the surface is depicted in accordancewith the present invention. Although the tips, whether created in asall-metal or metal coated tips or as thermoelectric tips have beendescribed thus far as being in contact with the surface opposite thetips. However, although the tips may be in contact with the opposingsurface, it is preferable that the tips be near the opposing surfacewithout touching the surface as depicted in FIG. 14. The tip 1402 inFIG. 14 is situated near the opposing surface 1404 but is not inphysical contact with the opposing surface. Preferably, the tip 1402should be a distance d on the order of 5 nanometers or less from theopposing surface 1404. In practice, with a thermoelectric coolercontaining thousands of tips, some of the tips may be in contact withthe opposing surface while others are not in contact due to thedeviations from a perfect plane of the opposing surface.

By removing the tips from contact with the opposing surface, the amountof thermal conductivity between the cold plate and the hot plate of athermoelectric cooler may be reduced. Electrical conductivity ismaintained, however, due to tunneling of electrons between the tips andthe opposing surface.

The tips of the present invention have also been described and depictedprimarily as perfectly pointed tips. However, as illustrated in FIG. 14,the tips in practice will typically have a slightly more rounded tip asis the case with tip 1402. However, the closer to perfectly pointed thetip is, the fewer number of superlattices needed to achieve thetemperature gradient between the cool temperature of the tip and the hottemperature of the hot plate.

Preferably, the radius of curvature r₀ of the curved end of the tip 1402is on the order of a few tens of nanometers. The temperature differencebetween adjacent areas of the thermoelectric material below surface 1404approaches zero over a distance of two (2) to three (3) times the radiusof curvature r₀ of the end of tip 1402. Therefore, only a few layers ofthe super lattice 1406-1414 are necessary. Thus, a superlattice materialopposite the tips is feasible when the electrical contact between thehot and cold plates is made using the tips of the present invention.This is in contrast to the prior art in which to use a superlatticestructure without tips, a superlattice of 10000 or more layers wasneeded to have a sufficient thickness in which to allow the temperaturegradient to approach zero. Such a number of layers was impractical, butusing only 5 or 6 layers as in the present invention is much morepractical.

Although the present invention has been described primarily withreference to a thermoelectric cooling device (or Peltier device) withtipped interfaces used for cooling, it will be recognized by thoseskilled in the art that the present invention may be utilized forgeneration of electricity as well. It is well recognized by thoseskilled in the art that thermoelectric devices can be used either in thePeltier mode (as described above) for refrigeration or in the Seebeckmode for electrical power generation. Referring now to FIG. 15, aschematic diagram of a thermoelectric power generator is depicted. Forease of understanding and explanation of thermoelectric powergeneration, a thermoelectric power generator according to the prior artis depicted rather than a thermoelectric power generator utilizing coolpoint tips of the present invention. However, it should be noted that inone embodiment of a thermoelectric power generator according to thepresent invention, the thermoelements 1506 and 1504 are replaced coolpoint tips, as for example, any of the cool point tip embodiments asdescribed in greater detail above.

In a thermoelectric power generator 1500, rather than running currentthrough the thermoelectric device from a power source 102 as indicatedin FIG. 1, a temperature differential, T_(H)-T_(L), is created acrossthe thermoelectric device 1500. Such temperature differential,T_(H)-T_(L), induces a current flow, I, as indicated in FIG. 15 througha resistive load element 1502. This is the opposite mode of operationfrom the mode of operation described in FIG. 1

Therefore, other than replacing a power source 102 with a resistor 1502and maintaining heat elements 1512 and 1516 and constant temperaturesT_(H) and T_(L) respectively with heat sources Q_(H) and Q_(L)respectively, thermoelectric device 1500 is identical in components tothermoelectric device 102 in FIG. 1. Thus, thermoelectric cooling device1500 utilizes p-type semiconductor 1504 and n-type semiconductor 1506sandwiched between poor electrical conductors 1508 that have good heatconducting properties. Each of elements 1504, 1506, and 1508 correspondto elements 104, 106, and 108 respectively in FIG. 1. Thermoelectricdevice 1500 also includes electrical conductors 1510 and 1514corresponding to electrical conductors 110 and 114 in FIG. 1. Moreinformation about thermoelectric electric power generation may be foundin CRC Handbook of Thermoelectrics, edited by D. M. Rowe, Ph.D., D.Sc.,CRC Press, New York, (1995) pp. 479-488 and in Advanced EngineeringThermodynamics, 2nd Edition, by Adiran Bejan, John Wiley & Sons, Inc.,New York (1997), pp. 675-682, both of which are hereby incorporatedherein for all purposes.

The present invention has been described primarily with reference toconically shaped tips, however, other shapes of tips may be utilized aswell, such as, for example, pyramidically shaped tips. In fact, theshape of the tip does not need to be symmetric or uniform as long as itprovides a discrete set of substantially pointed tips through whichelectrical conduction between the two ends of a thermoelectric coolermay be provided. The present invention has applications to use in anysmall refrigeration application, such as, for example, cooling mainframe computers, thermal management of hot chips and RF communicationcircuits, cooling magnetic heads for disk drives, automobilerefrigeration, and cooling optical and laser devices.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention, the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method of forming metal electrode pointed tipsof a thermoelectric device, the method comprising: forming a mask ofpatterned photoresist onto a layer of metal; etching the layer of metalin the presence of the photoresist mask to produce substantially pointedtipped structures of metal; and removing the photoresist.
 2. The methodas recited in claim 1, wherein the patterned photoresist forms an arrayof photoresist areas that correspond to areas for which tips of thesubstantially pointed tipped structures of metal are desired.
 3. Themethod as recited in claim 1, wherein the metal is copper.
 4. The methodas recited in claim 1, further comprising: coating the substantiallypointed tipped structures of metal with a layer of a second metal. 5.The method as recited in claim 1, further comprising: coating thesubstantially pointed tipped structures of metal with a layer ofthermoelectric material.
 6. The method as recited in claim 4, furthercomprising: coating the layer of second metal with a layer ofthermoelectric material.
 7. The method as recited in claim 1, whereinthe substantially pointed tipped structures are conical shaped.
 8. Themethod as recited in claim 1, wherein the substantially pointed tippedstructures are pyramid shaped.
 9. A system of forming metal electrodepointed tips of a thermoelectric device, the system comprising: meansfor forming a mask of patterned photoresist onto a layer of metal; meansfor etching the layer of metal in the presence of the photoresist maskto produce substantially pointed tipped structures of metal; means forremoving the photoresist; and means for coating the substantiallypointed tipped structures of metal with a layer of thermoelectricmaterial.
 10. The system recited in claim 9, wherein the patternedphotoresist forms an array of photoresist areas that correspond to areasfor which tips of the substantially pointed tipped structures of metalare desired.
 11. The system as recited in claim 9, wherein the metal iscopper.
 12. The system as recited in claim 9, further comprising: meansfor coating the substantially pointed tipped structures of metal with alayer of a second metal.
 13. The system as recited in claim 12, furthercomprising: means for coating the layer of second metal with a layer ofthermoelectric material.
 14. The system as recited in claim 9, whereinthe substantially pointed tipped structures conical shaped.
 15. Thesystem as recited in claim 9, wherein the substantially pointed tippedstructures are pyramid shaped.